One thing bothered me a lot in the movie, in the nordick mythology Loki and Thor were enemies right?
Loki did a lot of bad things and was destined to bring the Ragnarock, but in the movie they are brothers that still have a more or less good relationship, assuming that the nordic mythology from the cinematic universe is the same as the one in the real world where did those myths come from?
How did manking see Loki turning evil coming but Odin didn't?

Well, first of all even in the myths Thor and Loki has had numerous conflicts, but before the incident involving Baldur's death the Aesirs/Asgardians were very tolerant of Loki's mischief. He is noted as the god of Luck, of Lies, of Evil, of Mischief, and sometimes even of Chaos, but he has a place on Asgard like the rest of them.

If you really want to get a breakdown on Loki in the Nordic myths, look it up in the poem 'Lokasenna' (Loki's Quarrel), I think it was the last straw between him and Thor, and it's pretty characteristic of Loki from the comics too!

The fate of Loki in the myths is one of banishment - he ends up being trapped in the bowels of the Earth, with the entrails of his son binding him to a rock above which a serpent drips its poison on the fallen god. His wife, Sigyn, is with him, taking a cup to collect the venom so that it doesn't fall on his face, but everytime Sigyn leaves to empty the cup, and the poison falls on Loki, his jerks and screams cause tremors across the earth, causing earthquakes. By far, that's my favourite Nordic myth!

Ragnarok was something that all the gods knew about in the olden days, everyone spoke of it like we do about the Day of Judgment, so even if Loki existed in Asgard before the event with Baldur, Odin the all-knowing was very much aware of what would happen in the end. I suppose he kept him there because that's what he does - lets individuals play out their own fates. I think that's where this discussion would late, to the argument of fate.

If we may draw comparisons between the film and the Norse myth, we see that a lot of this is possibly happening before Loki gets banished. Maybe only Odin knows and no one else? There was that line in the movie where Fandral says "this isn't like Earth where you summon a little thunder and lightning and they worship you as a god...", and Thor himself seemed very much unaware of how things were on Midgard/Earth. So I'll hazard a guess and say that most of the Asgardians are not aware of our mythology, or the stories we have written about them. Only Odin knows. And he, allegedly, has a purpose for everything. Hope that helps?

Like WildcatNC says - the old myths come to us in parts, and not in chornological parts. The chronology is up to interpretation.

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THE JUSTICE BULLETIN published some of my thematic analysis on the symbolism in Nolan's superhero saga.
I call it Heroic Archetypes. You can read the parts on Batman Begins in the following links:
(pt 1; pt 2; pt 3; pt 4; pt 5; pt 6; pt 7)